Semiconductor fabrication may require carefully synchronized and precisely measured delivery of reactant gases to a process chamber. In an ALD (atomic layer deposition) process, for example, layered films may be created, one atomic layer at a time. Pulses of two or more precursor gases may be sequentially delivered to a process chamber maintained under vacuum. Each precursor gas may flow over a substrate surface to form an adsorbed monolayer on the surface. A second precursor gas may then be introduced into the chamber (after purging the chamber of the first precursor gas), and may react with the first precursor to from a monolayer of the desired thin film via a self-limiting surface reaction. A desired film thickness may be obtained by repeating the deposition cycle as necessary. The film thickness may be controlled to atomic layer accuracy by counting the number of deposition cycles. Likewise, pulsed deposition systems other than ALD processes may require a precisely measured delivery of pulses of gases to process chambers.
To obtain a high level of performance in an ALD process, or in other pulsed deposition processes, the delivery of pulsed mass flow of precursor gases into the semiconductor processing chambers may have to be measured and monitored, in a highly reliable and accurate fashion. ALD control techniques that are based on flow and pressure control may tend to be imprecise, due to various timing inaccuracies at the timescale of interest (milliseconds). Also, these techniques do not control total dose, but rather control flow or pressure, which can lead to significant dose variability due to temperature and timing effects.
It is desirable that highly repeatable and precise quantities of gases be delivered, for use in ALD processes and other semiconductor manufacturing processes. A method and system for actually measuring the amount of gas flowing into the process chamber, and for precisely delivering a desired number of atoms of precursor during each pulse, are desirable.